Micro­power

buffer? Wherever you put it, sooner or later you will want to load a program to that location. The only solution seems to be to use the screen RAM for temporary storage. The program therefore starts by clearing the screen; as it uses the margins as well as the ‘visible’ screen RAM, it also clears the screen at the end of the routine to restore the zeroes which the delineate the margins. A tally is kept on the screen of blocks which have been read correctly, and when all the blocks have been obtained the routine stops.

The revised Read incorporates the ‘load offset’ of Nas-Sys 3. A second argument can be used to force transfer of data from the buffer to memory, even when the checksum is wrong. This ensures that if you only have one copy of a program, and a persistent error on the tape, you don’t loose the whole block.

Because bad data is not written to memory, it is not necessary to use a separate verify command. To verify a tape you have just recorded you merely read it back with the R command – if the recording was faulty it will not corrupt the stored program. Therefore the routine does not test the value stored at ARGX (£0C2B), which is how the standard Nas-Sys routine distinguishes between Read and Verify. Consequently unless you change the address for the V command this will also read a tape into memory.